It is known that among various steel materials, austenitic stainless steel is susceptible to carburization. For example, a cracking tube, which is used for the thermal decomposition reaction in an ethylene manufacturing process of a petrochemical plant, is made of austenitic stainless steel, and carburization occurs on its inner surface after being used for long hours. Moreover, in the manufacturing process of the cracking tube, carburization occurs when heat treatment is performed in a poorly degreased condition. Since the occurrence of such carburization may cause a significant reduction of the life of the cracking tube, there is a need for accurately sensing whether carburization occurs or not.
For this reason, conventionally, an electromagnetic test such as an electromagnetic induction test is carried out on a cracking tube installed in a plant as a nondestructive inspection across the entire length of the cracking tube at the time of periodic maintenance of the plant so that whether carburization occurs or not is sensed based on the magnitude of the output value thereof. Moreover, also in the manufacturing process of cracking tube, whether carburization occurs or not is sensed by performing an electromagnetic test across its entire length or by cutting off both ends thereof and performing a microstructure observation.
In the above described electromagnetic test, it is a general practice that using a calculated calibration curve, which is a previously calculated correspondence between a carburized depth and an electromagnetic test output value, a threshold value of the electromagnetic test output value corresponding to a threshold value of the carburized depth to be sensed is determined in advance. Then, whether carburization occurs or not in a test material is sensed based on a magnitude correlation between the electromagnetic test output value obtained by performing the electromagnetic test on the test material and the threshold value of the electromagnetic test output value which is predetermined as described above.
In general, when calculating the above described calibration curve, first, a plurality of carburized materials each of which is expected to have a different carburized depth are prepared, and each carburized material is subjected to an electromagnetic test to obtain an electromagnetic test output value. Thereafter, each carburized material is cut off and subjected to microstructure observation so that an actual carburized depth of each carburized material, from which the electromagnetic test output value has been acquired, is measured. Thereby, it is possible to calculate a calibration curve which is a correspondence between the carburized depth and the electromagnetic test output value.
In this situation, since the carburized depth is affected by the manufacturing history or the usage history of a cracking tube, a plurality of carburized materials collected from one cracking tube having the same history may have an equivalent carburized depth. In other words, a plurality of carburized materials each having a different carburized depth may not necessarily be collected from one cracking tube conveniently to calculate the above described calibration curve. Therefore, it is a general practice that to increase the possibility to collect carburized materials having different carburized depths, carburized materials are collected respectively from a plurality of cracking tubes, each of which has a different history such as a manufacturing lot and a usage time, and are subjected to the calculation of the calibration curve.
As describe above, each carburized material to be subjected to the calculation of calibration curve is collected from the respective ones of a plurality of cracking tubes having different histories. Therefore, even if a plurality of cracking tubes having the same constituents and dimensions (outer diameter and inner diameter) in terms of the design specification are selected, and each carburized material is collected from each of the cracking tubes, the constituents and the dimension of the base metal of each carburized material may vary. As a result, the electromagnetic properties (electric resistance, etc.) of the base metal of each carburized material may vary.
In an electromagnetic testing such as an electromagnetic induction testing, it is a general practice that an alternating current of from several hundreds of Hz to several tens of kHz is applied depending on the wall thickness of the material to be measured such that the penetration depth of the alternating current is several times of the wall thickness. Therefore, even assuming that carburization has not occurred in each carburized material, the different electromagnetic properties of the base metal of each carburized material lead to a different electromagnetic test output value of each carburized material. That is, the electromagnetic test output value when the carburized depth is 0 μm (the reference point) will vary from one carburized material to another. Thus, the accuracy of the calibration curve, which is calculated by using the electromagnetic test output values for which the reference point varies from one carburized material to another, will be reduced according to the amount of deviation of each reference point. As a result, a problem arises in that the accuracy of the threshold value of the electromagnetic test output value, which is predetermined by using the calibration curve as described above, will also be reduced, further leading to a decline of accuracy in sensing whether carburization occurs or not.
Although various methods for sensing whether carburization occurs or not have been proposed including ones which are not in the actual use yet (for example, see JP3-253555A, JP62-6153A, JP4-145358A, JP6-88807A, JP2000-266727A, JP2004-279054A, and JP2004-279055A), none of these method is able to solve the above described problems.